1. Field of the Invention
This invention relates to a process for depositing aluminum oxide coatings on a flat glass substrate. More particularly, this invention relates to an atmospheric pressure chemical vapor deposition process for producing aluminum oxide coatings at high growth rates on flat glass using a coating precursor gas mixture comprising an inorganic aluminum halide and an organic ester having a β hydrogen on the alkyl group bonded to the carboxylate oxygen.
2. Summary of Related Art
Due to a number of desirable physical and chemical properties, aluminum oxide coatings have been used, primarily, in connection with diffusion barriers for species such as alkali metal cations, oxygen and water, passivation layers in microelectronic devices, wear resistant coatings, and corrosion-resistant coatings.
Growth of aluminum oxide coatings by CVD has been reported in the scientific literature. Deposition from AlCl3 has been performed under atmospheric pressure (and low pressure) conditions using CO2 and H2 as a hydrolysis source (Silvestri et al., J. Electrochem. Soc. 1978, 125, 902 and Park et al., J. Vac. Sci. Technol. A 1983, 1, 1820). Deposition was carried out at 700–900° C. and produced low film growth rates between 0.2 and 2.2 Å/sec.
Highly reactive organometallic precursors such as alkyl aluminum compounds, R3Al, have been used to grow aluminum oxide coatings. Other methods use alkoxides such as Al(OR)3, or mixtures of both alkyl aluminum and aluminum alkoxides, i.e., [R3Al2(OR)3], and adducts of alane, D:AlH3 where D is a Lewis base. Other less used aluminum oxide precursors have been aluminum-based esters, for example, [Al(OOCR)3], Al(N(TMS)2)3, where TMS is trimethylsilyl, and Al(BH4)3. Each of the above-described chemistries has certain drawbacks with low growth rates dominating the results.
Deposition of aluminum oxide films has also been disclosed in the patent literature. For example, U.S. Pat. No. 2,805,965 describes a process for applying a film of a metal compound to a surface. The patent states the vapors of an organic hydroxy salt of, for example, aluminum, can be used to form a film of aluminum oxide on a surface. In one example, aluminum ethylate is boiled under pressure. The vapors from the aluminum ethylate are fed into a tube through which a copper wire is passed. An electric current is passed through the wire to heat it. As the ethylate vapors reach the wire, a film of aluminum oxide deposits on it.
A process for forming a metal oxide on a substrate is taught in U.S. Pat. No. 4,129,434. According to the patent, a solution of at least one metal compound is applied to the substrate where the solution is a metal acetyl acetonate or a mixture of metal acetyl acetonates. The metal acetyl acetonate solution may include aluminum. The solution can be sprayed onto the glass ribbon located in the drawing chamber of a glass drawing machine, or in an annealing lehr, to form a metal oxide film.
U.S. Pat. No. 4,160,061 teaches dissolving an acetyl acetonate of aluminum in an organic solvent together with at least one similarly decomposable metal compound to form a metal oxide on a substrate. The decomposable metal compound is selected from the compounds of Cr, Co, Ni, Fe, Zn, Sn, Cu, Mn and Ti. In one example, acetyl acetonates of Cr, Co, Fe and Al were dissolved in a mixture of dichloromethane and methanol with various concentrations of aluminum acetyl acetonate. Each solution was sprayed onto a glass substrate and heated in an electric furnace to 630° C. A metal oxide was formed on the glass surface in this manner.
A method for forming an insulating layer on an active matrix display is provided in U.S. Pat. No. 5,374,570. First, a substrate is exposed to an aluminum chloride vapor. This exposure results in a film of the vapor being deposited on the substrate. The substrate is then moved from the aluminum chloride vapor to a water and/or oxygen vapor environment. A film of water and/or oxygen vapor forms on the substrate. The substrate is then moved back to the aluminum chloride vapor where the aluminum chloride is converted to a very thin layer of aluminum oxide on the substrate. This process is repeated thousands of times to form the aluminum oxide layer.
U.S. Pat. No. 5,648,113 discloses a process for chemical vapor deposition of Al2O3 on a substrate using aluminum tri-isopropoxide (ATI). A source of ATI is heated to a temperature sufficient to provide a flow of chemical vapor through a vapor delivery system. The vapor is combined with a preheated inert gas, such as argon. The vapor and inert gas are injected into the processing chamber where the chemical vapor in the gas mixture reacts on the substrate to form an aluminum oxide film thereon.
It would be desirable to form aluminum oxide films at essentially atmospheric pressure and to produce them at deposition rates compatible with time-critical manufacturing processes, for example, production of flat glass by the well-known float method. Those skilled in the art have continued to search for a method of producing aluminum oxide films meeting the above-noted criteria in order to have available, affordable films for optical thin film stack designs.